This invention relates to wastewater treatment, and more particularly to an efficient on-site wastewater treatment and disposal method and apparatus utilizing filtration.
Various systems have been used for treatment of residential and other wastes. In cities there have typically been central sewage treatment facilities for water-borne solid and liquid wastes conducted thereto from residences and other sources. Generally these systems have included separation of the solids from the liquids, sometimes by filtration. However, separation at this stage is only partially effective, since the solids and liquids have become nearly homogenized in transit. Such "separated" solids have often been subsequently subjected to a drying operation. See, for example, U.S. Pat. Nos. 2,736,434, and 3,559,807.
The liquids from such sewage treatment, which at this point carry many substances from the solids, have usually been chemically treated to the point that they can be returned to the soil or to flowing water. The solids have generally been either buried or composted. However, composting of sewage solids at this stage is nearly always anaerobic, since anaerobic decomposition has already begun as the wastes travelled through sewage lines to the treatment facility. Anaerobic composting is often not as desirable as aerobic compositing, primarily because of strong odors and the contamination of the surrounding air with disease-carrying organisms.
One type of on-site sewage filtration apparatus has been suggested in U.S. Pat. No. 3,747,766. That apparatus was intended to separate solid wastes from liquid wastes soon after their exit from households or other sources. However, the system of the patent was completely closed, with no provision for aerobic composting of solids. The final treatment of the solids was to be dehydration and/or burning. This type treatment is not only costly but also wasteful of the soil conditioning product that can be derived from aerobic composting.
The aforementioned U.S. Pat. No. 3,559,807 discloses a conveyor-type sewage filtering device which is in some respects similar to a filtering apparatus of the present invention. However, the filtering device of that patent does not include several particularly advantageous features of the present invention, and it is shown for use with solid material drying and burning apparatus.
There has been a need for an efficient, compact, and reliable wastewater separation and treatment apparatus in several areas of application in addition to sewage treatment. For example, in the food processing industry large quantities of water bearing organic materials must be separated from such materials at least to the point that the water can be safely returned to the soil. Similar problems exist with other industrial water-borne wastes. In industrial air pollution reduction equipment, solid pollutants are often ultimately carried by water. This water should be separated from the solids and purified before it is introduced to the soil or flowing water.
One aspect of the present invention concerns liquid level sensors for sensing the level or presence of liquid in a vessel or conduit. In the past, electronic level sensors responsive to variations in electrical capacitance introduced by proximity of a sensor and a liquid were known.
Such conventional capacitance responsive sensors utilized a variable capacitance element as a sensor or probe in combination with e.g. an alternating current bridge circuit. In such circuits essential components included a very stable reference oscillator, an adjustable bridge network, a rectified circuit, and a sensitive amplifier. Such circuits were characterized by complexity, high cost, lack of stability in rugged industrial environments, and generally unacceptable reliability.